Frequently it is desired to generate variable width pulses with a minimal amount of circuitry. For example, in certain liquid crystal display (LCD) devices, digital-to-analog converter circuits are formed using variable width pulse generators. Display information represented by binary numbers are converted into variable width pulses which pulses are used to control the conduction times of switching transistors. A ramp voltage is applied to one electrode of the switching transistor, the other electrode being coupled to a display element. The transistor is conditioned to conduct at predetermined intervals and is turned off as a function of the variable width pulse. At the time the transistor is turned off, the ramp voltage, and thus the potential applied to the display element is proportional to the binary value controlling the pulse width. (See for example U.S. Pat. Nos. 4,742,346 and 4,766,430 Gillette et al. which are incorporated herein by reference.)
Display devices of this type may include many hundreds of such digital-to-analog converters and thus many hundreds of variable width pulse generators, requiring that such circuit elements be parts efficient. In addition, the circuitry may be fabricated with material having low carrier mobility such as amorphous silicon. In this latter instance special circuit configurations are necessary to overcome speed limitations incurred by the low carrier mobility.
Typically variable width pulse generators are implemented with programmable counters. Consider that the picture information is represented by eight bit binary words, and that the maximum variable pulse width is 50 .mu.sec (approximating the active portion of a video line). The incremental variation of the pulses is 50/2.sup.8 .mu.sec or approximately 0.2 .mu.sec. To realize this resolution with a counter type variable pulse generator requires a counter clock frequency of 5 MHz. However, it is not currently possible to fabricate a practical programmable counter that will operate at 5 MHz with amorphous silicon. Thus there is a need for an alternative type of variable width pulse generator.